1. Field of the Invention
The invention generally relates to the field of signal processing for wireless communication. More specifically the invention is related to spread spectrum interference cancellation.
2. Discussion of the Related Art
In order to efficiently utilize time and frequency in a communication system, multiple-access schemes are used to specify how multiple users or multiple signals share a specified time and frequency allocation. Spread-spectrum techniques may be used to allow multiple users and/or signals to share the same frequency band and time interval simultaneously. Code division multiple access (CDMA) is an example of spread spectrum that assigns a unique code to differentiate each signal and/or user. In a typical CDMA wireless telephony system, a transmitter may use a combination of scrambling codes and covering (i.e., orthogonalizing) codes. For example, each transmitter may be identified by a unique scrambling code or scrambling-code offset. For the purpose of the exemplary embodiments of the invention, a scrambler (which is typically used in a W-CDMA system to scramble data with a scrambling code) is functionally equivalent to a spreader, which is typically used in CDMA2000 and IS-95 systems to spread data using short pseudo-noise (PN) sequences.
A single transmitter may transmit a plurality of signals sharing the same scrambling code, but may distinguish between signals with a unique orthogonalizing code. Orthogonalizing codes encode the signal and provide channelization of the signal. In W-CDMA, orthogonal variable spreading factor (OVSF) codes are used as multiple-access orthogonalizing codes for spreading data. CDMA2000 and IS-95 employ Walsh covering codes for multiple-access coding. These codes are typically designed to have minimal cross-correlation to mitigate interference.
A receiver discriminates multiple received signals by exploiting the properties of spreading and scrambling codes applied to transmissions for each user. The receiver attempts to temporally align the codes of a desired signal with a locally generated replica of spreading and scrambling codes for a particular user. However, the downlink cellular channel typically introduces multipath propagation, which destroys the orthogonality between Walsh channels and produces Multiple-Access Interference (MAI). Even relatively slight multipath effects can introduce cross correlations between codes and cause CDMA systems to be interference-limited. MAI also arises from interfering base stations.
Interference can degrade communications by causing a receiver to incorrectly detect received transmissions, thus increasing a receiver's error floor. Interference may also have other deleterious effects on communications. For example, interference may diminish capacity of a communication system, decrease the region of coverage, and/or decrease maximum data rates. For these reasons, a reduction in interference can improve reception of selected signals while addressing the aforementioned limitations due to interference.
Interference cancellation attempts to suppress intra-cell and inter-cell MAI by estimating and removing the contribution of each interferer from the received signal. Because the capacity of CDMA systems is MAI-limited, estimating and canceling the MAI can increase system capacity. Furthermore, interference cancellation can reduce the signal-to-interference-and-noise ratio (SINR) required to achieve a specific symbol-error-rate (SER) or data throughput for a specific number of users, thereby increasing battery life and decreasing transmission power, increasing the communication range, and/or increasing network capacity.